CA1110726A

CA1110726A - Detection system

Info

Publication number: CA1110726A
Application number: CA301,268A
Authority: CA
Inventors: John D. Mccann
Original assignee: Parmeko Ltd
Current assignee: Parmeko Ltd
Priority date: 1977-04-28
Filing date: 1978-04-17
Publication date: 1981-10-13
Also published as: US4303910A; GB1604219A

Abstract

ABSTRACT

This invention is directed to a system for monitoring the position of a receptor reradiator in a surveillance zone. It comprises first means for transmitting a first signal through the zone, a receptor reradiator operable in response to reception of said signal to radiate at least one reply signal which is a function of the first signal and of the position of said receptor reradiator in the zone. It also comprises a receiver for re-ceiving said reply signal, means controlled by the receiver in dependence upon the reply signal to indicate the position of the receptor reradiator in the zone and an alarm triggerable by the receiver responsively to the latter receiving the or one of the reply signals.

Description

~3 ~he în~ention relates to detection systems for monitoriag the position in a checking zone of an artîcle~ and to passive marker ~ags for such ~y~tems,.

Detectio~ systems for detecting the pre~ence in'a checking zo~e of an article are primarily used in store~ and warehouses for detecting so far as is possible, the unauthorised removal of articles~ For thi~ purpose a checki~g 20ne i5 established for example in a store which ; can be said to be do~nstream of cash paying points.
Each article on sale in the store is provided with a tag which in the normal cour3e of event~g is removed at the paying point but i~ not so removed, its presence in the detection zone operates an alarm~ ' Various systems are in use and these broadl~ fall into two main c~tegories namely magnet,ic and radio frequency s~s~ems. With magnetic systems the tag incorporates magnetised material the presence of which in the detection zone is detected b~ magnetic monitoring equip~ent. This type of system has the disadvantage that the monitoring equip~ent must be very carefully ad~usted otherwise it will either not provide an alarm when required to do so or it may provide a false alarm due to metallic objects normally carried by a person~ disturning the magnetic field.

Radio fre~uency systems can be made more sensitive and also reliable and one such system employs a tag having ,. - 2 " ., ., .., :. : ,: , .. .
.,, .:, ~. ~:

1.~

2~
electrical components thereon which pick up energy radiated from a transmitter and by means of a non-linear element, re-radiates the energy at twice the frlequency of the received radiation. A receiver is provided which is tuned to the frequency of the reradiated signal and when such a signal is detected, an alarm is given. One problem with such 8 system if the fact that the transmitter may go out of adjust~ent and radiate a second harmonic signal which will be detected by the receiver and ther~by will provide a false alarm. Other faults with such a system can occur~
, ~he present invention seeks -to provide a detection system which is relatively simple and conveient to use and is less susceptible to triggering by extraneous signals.

.
~he present invention also seeks to provide a passive marker tag for such a system, and also a method of monitoring the position of such a tag in a surveillance zone.

The invention provides in its broadest aspect a system for monitoring the position of a receptor re radiator in a surveillance zone, characterised by first means for transmitting a first signal through said zone9 a receptor reradiator operable in response to reception o~ said signal to radiate at least one reply signal which is a function of said first signal and of the position o~ said receptor reradiator in the zone, a

- 3 ..~ .

, "' ': :' ' ' receiver for receiving said reply signal~ means control led by the.receiver.in dependence upon said reply signal to indicate the position of the receptor reradiator in the zone, and an alarm triggerable by the receiver responsively to the latter receiving the or one of the reply signals.

.
-~ ?he invention provides in another of its aspects a receptor reradiator for a system characterised by a first aeria~
means for receiving said first signal, second aerial means for radiating said reply signal, and a non-linear element coupling said first and second aerial means.

The invention provides in yet another of its aspects a method of monitoring the position of a receptor reradiator in a surveillance zone, characterised by radiating a first signal through said zone; detecting in said zone the presence of at least one repl~ signal whlch is a function o~ said first signal and of the position of the receptor reradiator i.n the zone in dependence upon said reply signal indicating the position of the receptor reradiator in the zone, and triggering an alarm responsively to the detection of the or one of the repl~ signals.

The present in~ention is ~rther described hereinafter, by way of example, with reference to the accompan~ing drawings, in which:-' .;........ ,:
. . ~ . .
:. . : , ~. : , :, - -: , ~
.

;-- .
~igure 1 is a schematic diagram o~ one emb~diment of a system according to the present invention;

~igure 2 is a circuit diagram of a typical tuned diode receptor reradiator for the system of Figure 1;
.' :
Figure 3 is a ~chematic diagram of El second embodiment . of a system accordi~g to the present invention;
,~ ''' ` ' . . .
~igure 4 is B circuit diagram of a receptor reradiator for the system of ~igure ~; and .. ... . . . . . . ..

.. . . ..
.' '~, ' ' ' 7 ~ ' . , ' ~ , , ., , '.

~'. ., ' , '~, ' ' ' .
` 1 ,. " "~ ' ' ~ ' ,` ~ ; ' t~
~ 6 Figure 5 i~ a circuit diagram of u modification for part of the sy~tem of ~lgure ~.

~he sg~tem illu~trated in Figure 1 uti.li~es two tran~matter~
10~ 11 which operate in the ~.W. or V.H.F~ part of the radio ~requency bands~ ~he tra~mitters are coR~ected to ~eed respective aerials 12~ 13 which are disposed i~.
or adJacent a detection zone which is indicated at 14 and are arran~ed to transmit their respective signals through the zone 14~

~he zone 1~ may include a conveyor on which merchandise 10 travels or may define an aisle or doorway in a department store or the like through which customers must pa~s.
The zo~e 14 may eve~ be a room7 the system being set to activate any receptor reradiator carried b~ articles of merchandise in the room.

15 A marker tag 18 which is normally attached to an article of merchandise carries a receptor reradiator,suoh as is shown in Fi~ure 2,which includes a $uned resonant circuit 19 tuned to receive the two sig~als from the transmitters 10, 11, a non-linear device in the form of a diode 21 20 and.a tuned reradiator circui$ 20. An aerial 15 of a receiver 16 is also located in or ad~acent t e zone 14 and is tu~ed to receive signals radiated by the tuned circuit 20. 0~ receptio~ of s~ch signals the recei~er 16 triggers a warning device 17 which may be audible 25 visual or both audible and visual~

.. _ -.

.

\r~26 ~he fundamental frequencie~ f1 and f2 to which th0 two transmitter~ 10, 1~ are respectively tuned9 differ by a relatively small amount as compared with the magnitud~
of the frequencies~ In a particular example the ~requency 5 - f1 f transmitter 10 is 27.0 ~ whilst the freque~cy f2 of the transmitter 11 is 27~2 ~ ., An ~lt~r~ative ch~ice ~or the fundamental frequencies is ~pproximately 450 M~o The tuned circuit 1~ of ~he tag 18 i~ tuned to a ce~tre 10. frequency fc which is substantially midway between the two transmittcr fundamental frequencies, i~e the sum of the transmitter frequencies divided by two fc = (f1 ~ f2)/2O
~he bandwidth of the tuned circuit 19 is also designed sufficiently wide to include the two transmitter frequ0ncies without introducing any serious reduction in received signal strength. ~he tuned circuit 19 is coupled tc the tuned circuit 20 by the diode 21. ~he latter i~
merely one example of a non-linear device whic~ may be used and which utili~es the well known fact that the non-linear response of such a device to received signals of different frequencies gives rise to sum and difference frequencies, known as inter modulation product~, as well as harmonics~ With recei~ed frequencies of f1 and f2 (in the particular exa~ple ?7.0 MHz and 27.2 M~z) the diode 21 generates the following ma~or inter modulation and harmonic.frequencies = 2f1 (54 ~ )~ 2f2 (54~ MHz), f~ + f2 (54.2 MHz) and f2 ~ f1 (0.2 ~ )~

. ~

: :

~he tuned circuit 20 iB tuned to a ~elected inter modulatio~
product, in the particular example ~.2 ~ and radlates this sigQal to the receiver aerial 150 ~hus~ if a tag 18 is brought into the detection zone the radiated ~ignal from the tag is detected by the rece:iver 16 which then trigger~ the.warning device ~7, the receiver 16 being tuned to the radiated sig~al frequency (54.? ~ ~ with sufficient selectivit~ to preclude triggering of the ~arning device 17 b~ ad~acent signalsO

. 10 ~he tag.18, however, is also designed to radiate one or ~oth of the second harmonics 2f1 an 2f2 f the transmitte~
fund~mental freguencies to enable the position of the tag 18 in the detection zone to be ascertained.
R~d~ation iB effected by the tuned circuit 20 where the latter is tuned to 54.2 ~ , or by a further tuned circuit7 not shown, where the difference between selected inter modulation product and the second harmonic i~
- sufficientl~ great to warrant it. (The or both second harmonics may alternativel~ be used to activat~ the warning device, if desired, although this does increase the risk of false alarms)O

As shown in Figure i the aerials 12 and 1~ are loop aerials (equally dipole aerials can be utilised although these lack the directional characteri~tic~ o~ loop aerialsO In the ca~e of the loop aerial the diameter - of the loop would be in the order of one metre~ which . . . , : . .. . .
,. ;.
. . . .
., . . , : :

'726 are ~eparated from each other~ as ~hown9 ~o a~ to produc~
in tha detection zone a variation in the ~ield ~trength of the signal radiated from each tra~smitter. Clearly . i~ the centre of the detectio~ zone the ~ield of the - 5 ~ignals f1 and f2 prefera~ly ~hould be the same but towards the fri~ges o~,the zo~e m~ving in the directio~
of the aerial~, the field stre~g~h o~ the ~ignal radiated ~rom one transmitter will i~crea~e, whi~st at the sa~e tim~ th~ field ~tre~gth of the signal radiated from th~
other transmitter will decreaseO .~berefore~ the amplitude~
Or the seco~d harmoni G signal~ radiated b~ the ~ag 18 will var~ a~ the signal strength of the sigpals received by the tuned circuit 19 ~rom the transmitter vari~s.
~his fact is utilised by.the receiver 80 that whil~t it causes the w~rning device 17 to operate when a ~ig~al correspondi~g to the sum. of the transmitt~r ~requencies i~ obtained, it al60 provide~ an output respon~ive ~o the harmQnics of the transmitter frequenciesO Comparison of the relative strengths of thes~ further signal~ provides an indication of the position of the tag 18 in the detectio~ ~one~ Where the zone 14 is a doorway, ~or example, the transmitters may be placed on respective sides thereof~
Where the zone i8 an aisle the tra~smitters may be placed at respective ends thereof.
.

25 I1D order to provide further saf~guard~ against fal~;e alarms, one or both o~ the transmitters' 10~ 11 radiated fundamental ~reque~cie~s ma~ be modulated a~d this modulation will: .

_ g .~ . - ~ . , .
- . ~ , .. .

.,,,_,, , ,, . ' ' ' , .
:` ".' ~:
.. '', ' . .
,:
: , ' , v appear in the signal~ received at the receiver~ The ~ignals can be demodulated in the re~ei.ver and comp~red with the original modulating ~ignal or ~ignal~ to determine whether the signal arriving ~t the aerial 15 ha indeed originated from 8 tag which is in ~he detection zone~
Alternatively triggering of the warning device 17 may be effected only when the receiver receives two or more of the inter modulation products simultaneously~

Where one of the fundamental frequencies is modulated, 10 what is known as the cros~ modulation effect will also give rise to radiation by the tag 18 of the second harmonic of the other lundamental frequency but with the modulation imposed thereon. ~he depth of modulation will vary with the distance of the tag 18 from the 75 modulated and unmodulated transmitters and the depth of modulation therefore provides an additional indication of the tag position.

Although the receiver and the circuit 20 are tuned to the sum of the fundamental frequencies of the tra~smitters 20 for the purpose of triggering th~ warning device 17 this purpose may be served by any one of the i~ter modulation products~ ~or example, it is possible for the receiver and circuit 20 to be tuned to the di~ference frequency i.e 0.2MH~.

25 By using the radio frequency bands the system hereinbefore described ha~ the advantage over a system which uses a single microwave frequency that the electronic circuitry of the ~ ,.. '''. ' ::
... ~ . , -.~
. .
. :

recsiYing ~nd transmitting ~ection~ iB ~impler~ ~nd there iB les~ shielding of the marker tag~ ~y persons carrying articles being protected~ Whilst in F~he particular example hereinbefore described the f~damental frequencies are 2~.0 and 27.2 M~z~ this adYantage may be obtained with fundamental frequencies up to about 100~M~z~

The resonant circuits on the tag may be in the ~orm of tu~ed loops, or if spac~ permits, similar to a folded dipoler It should be remembered that it is necessa~y for the tag to be affixed to a sa~es article and therefore it needs to be comparatively small, for example, about 100 mm x 25 mm x 3 mm thick~ At the ~ame time howe~er it should be resistance to be~ding and also abrasion. A convenient material i~ a copper clad glass fibre laminate of the type used in the manufacture of printed circuit boards providihg some form of coating is applied, for ex~mple a plastics material, or providing the material forming the track is suitabl~ re~ista~t to abrasion. Other forms of laminate can be used providing suitable protection is pr~vided and the non-linear device may be a ~unction of materials which exhibits a non~
linear current/voltage relationship at the operating frequency.

A number of different examples for the constructio~al details of the marker tag 18 are described below~

q'he resonant circuit~ are formed by printing thin al~minium ~r copper conductors onto a substrate, speci~ic - examp1es being stiff cardboard or plastics sheet to form inductance coilsO E~ch coil i~ ~uned to the appropriate appropriate frequency by pl~cing a ~ir of thin met~l ~ilm conductor~ on oppo~ite side~ o~ the substrate to form a capacitor~ the substrate forming the dielectricO
, .

The non-linear element comprises a metal to se~l-. 5 conductor combinatiou and ~pecific examples ~re:

a) cuprous oxide semî conductor con~ected betweenpair of copper electrodes~

b) cuprous sulphide on cadmium ~ulphide ~emi-co~ductor connected between a pair o~ copper electrodes 3 c) selenium semi-conduc-tor connected between a pair ; o~ copper electrodes, d) titanium dioxide semi-conductor connec-ted between a titanium electrode and a silver electrode,.

e) . lead sulphide semi-conductor connected between a pair of copper or aluminium electrode~, f) magnesium oxide semi-conductor connected between a magnesium electrode and an aluminium electrode, g) aluminium (h1203) semi-conductor connected between . a pair of ~luminium electrode~ .

h) zirconia (5rO2) on zirconium connected between aluminium electrode~
: - ~2-, ~r~z6 i) g~llium ~rsenide semi-conductor connected b~tween a pair of gold or alumi~ium electrodes.

~he non-linear element i~ for~ed onto the sub~trate a~
~pecific examples of the process for achieving tbis are:

i) screen printing the layers, ii~ chemical formation of oxide and sulphide at elevated temperatuI~e~
iii) formation of oxide layers by electrolysi~ (for example~
form~tion of alumina layers), iv) sputtering, v) evaporationO
.
In order to control the capacitance vf the ~unction of the ~on-linear element, -the area of the ~unction.is controlled by a photo-lithographic process, by usi~g a small mechanical press tool, or by using a pulse from a laser to form a contact over a small area.

An improvement in the positional definition o~ the above describec system can be obtained if more than two transmitters are emplo~ed~
~or exa~ple if three transmitters are employed then whilst there are three sums of the three fundame~tal transmitter frequencies, it is likel~ that only two of these would be emplo~ed to give an i~dication of the appro~imate location of the tag within the detection zone.

~ ~3 -, . :.
. , ::

2fi A system using thre~ transmitter~ i8 illustrat~d i~
~igure ~ where the illustrated system US88 two ~eparate transmitter~ 30, 32 in the so-called inductio~ band (16 to 150 ~Hz3 together with ~ third tran~mitter 42 operatiDg i~ or near the microwave bandO ~he transmitters 309 32 are placed at spaced ~art locations in the zon~
34 to be surveyed and are preferably at ext.reme locations in the-zone, for example on respective sides thereof where the zone is a doorway and respectivel~ ad~acent th~
entrance to and exit from the zone where the latter~is an aisle. Suitable frequencies f~r the transmitters are, for example, fa ~ 130 XHæ for transmltter ~0 a~d fb = 80 ~H~
for transmitter 32. ~ignals at these frequencies are radiated through the zone 34 by9 for example, inductively loaded rod-like aerials 36, 38, or loop (i.e. conti~uous) aerials, excited by the transmitters to produce high streugth electric and magnetic fields in th~ zo~e 34.
The aerials may of course be located at the extremities of the ~one 34 while the transmitters are remote.t~erefrom and coupled to the aerials by suitable means.

~he system of transmitters and associated aerails may b~ arranged either side of a doorway so to survey horizontally across the protected zone~ or the items of system hardware may be arranged to survey vertically~
p~eferably downwards over the zone to be protected, thus leaving-the floor area unobstructed~

Since the cost and size of a passive receptor reradiator tag7 such as tag 409 ~USt be as small as practicable~
such considerations ruling out the tag being capable _ 1.4 .
.. , ~

.
of operating directly at~the induction ba~d ~reguencie~, third higher frequency fc i~ prc)vided as ~ csrIier for îrequenci~R f~ and fb., The, frequenc y fc is ~xansmitted - throilg~ the zoIIe ~4 as electromagnetic radiation from .
5 . the third transmitker 42, the freguency being ch~se~ for exa~ple at 900~. ~he tag gO again i~cludes a non-linear device, pref erably a diode 44 " but the tuned circuiti8 199, 20 o~ the tag are :ceplaced by a half w~e dipole aerial re~onant at freguenc;sr ~ ~900 X~Z~O The diode ~ i~
preferably offset from the electrial centre of the aerial to.i~crea~e the effecti~eness of the ~ield picked up from the i~duction band transmitter~ ~0, 32.

~he transmitter 42 preferably has two aerials 44, 46 located at oppo~ite ends of the æone 34 to provide a more u~iform diatribution of electromagnetic radiation at 900 ~ throughout the ~ne.
,., ,' ' ..~ ' Two receiver aerial~ 48, 50 tuned to 900 ~ are also located at opposite e~ds of the z~n~ 34 to receive ~igpals reradiated from the tag 40. The receiver aerials ars coupled to a mixer 52 to whi~h the t,ransmitter 42 `
also feed~ ~ greatly atte~uated signal at the carrier freque~cy fCD ~he attenuation can be ~fected i~ the transmitter, in the mixer 52 or in the liDk between the two but is such as to enable the mixPr tQ mix thi& attenuated ~ignal with ~ignals from the aerial~
48 and 50 to~a~e ~he carrier compo~ent fc~rom the latter ~ignal The atte~uated ~ignal beats with the carrier co~pone~t to produce - a zero best ~reque~c~ signal.~
,~ , . ~ . . ..
_ 15~
, . ` .
, ~

` . ~ - .

~ ~ o When a tag 40 is present i~ the volume 34 ~nd thu~
receiving 8ignal8 at the f-requencie~ fa~ ~b and then provided the field strength of at least o~e frequency compone~t is su~ficient~inker modulation of the low and high frequency ~ignals will occur in ~he non-linear device$ i~e. the carrier f.reque~cy fc will be modulated by the two i~duction band frequencies fa a~d fb. Generall~ for external i~ter modulation to occur the field strength Or at least one of the frequency components fa~ fb and fc ~ust exceed 9~Y per me~re in the region of the ~on-linear device~

, Once this threshold is exceeded the intensity of the inter modulatio~ products varies in dependence on the field strength3 of the incident frequency components. In the - 15 present example the inter modulation products are as follows:

f ~ f (in the particular example .900O13 c _ a and 899.87MHz) c - ~b (899.92MHz and 900.08MH~) fc + (fa + fb) (899089~Hz and 900.21MHz) ~c + (fa ~ fb) (899~95~Hz and 900.05~z) The signal~ at frequencies fa~ fb (fa ~ fb) a ( a b have thus become upper and lower sideba~d~ on the carrier signal fc~
_ 16 . . _ . ... ~ , . ~, , . , . , ..
- :

~ ~07 ~d6 I~ the signal ~trengths of the compo~e~t~ f~7 ~b and r~
greatly exceed the threshold valu0 then additional inter modulation products are ~enerated ~ ~ollows:
.

c a c _ b fc ~ 2(fa ~ fb3 c ~ 2(fa ~ fb) 2fn ~ fb , ~c ~ ~fb ~ ~a .etc.

In addition, the second harmonie 2fC of the carrier frequenc~ may be Kenerated with the above sidebands.

Figure 4A shows a more sensiti~e form of marker tag to that shown in Figure 4~ .

A coil OI moderate 'Q~ with all area of approximately 2 cm2 and flat profile i5 inserted betweeD the diode and, (preferably), the shorter of the two antenna arms. ~o increase the effective arlea of the coil without cha~ging physical dimensions, a piece Or ferrite or other suitable material may be employed as core matexial~ Also to 20 maintain the 900 ~ aerial ~t resonance, the tip to tip dimension ~hould be reduced below hal~ wavelength to compensate for the bulk of the coil and associated capacitor .

~he coil i~ made to re~onate ut a frequency approximately mid-wsy between fa and fb by shunting it with capacitor C0 ~he capacitor i~ preferably of the ceramic block type ~o that ~ low impedance may be presented to the 900 MHz - 5 current flowing simultaneousl~ in the ante~na system.

.
~he low freque~cy vol~ages induced i~ the coil ~rom the loop aerlals are thus added in s2ries with the 900 M~
compo~nt picked up by the antennaO The combinatio~ of these voltages impressed on ~ non-linear device causes i~ter modu~ation of the transmitter fre~uencies i~
the manner described earlier.

Apart from the signal voltage gain associated with the 'Q' of the coil, the voltages induced via magnetic coupling are less affected by the screening properties of certain types of merchandise~

The external in$er modulation products generated i~ the tag 40 are reradiated and picked up by the receiver ~erials 48, 500 ~he mixer 52 mixes these signals with the attenuated carrier signal from the tra~smitter 42, thus separating the carrier frequency from the i~ter modulation products. The output from the mixer 52 thus contains signals at frequencies Xa~ fb~ (~a ~ f~) and (fa ~ fb)- these being the most prominent.

The receiver 53 in the described embodiment selectively amplifies the fir~t three of the abo~e ~ideba~ds (the number of the Yidebands chosen ~or selective amplification _ 18-, , ,, .- ~;

may of course be varied as m~y be the ~ctual sideb~nds chosen) in three.separate channels~

- Each cha~nel includes a respecti~e filter 60, 62, 6 to which the output of the mixer 52 i.s co~}nected.

The three filters are narrow pasi~ barld Iilters with centre freque~cies resp~ctivel~ at the sideband frs~uencies~
the ~ilters ser~ing to separate the three chosen sid~bands ~nd filter our any remaining and unwanted sig~als at the mixer output. Each filter 60, 62g ~4 is connected via a respective amplifier 66, 68~ 70 to a level d~tector circuit 72, 74, 76 of a logic circuit 55, each level detector circuit being, for ex~mple~ a Schmitt trigger designed to respond to a relatively low level input signal to switch its output from a logic 1 to a logic signal~ Input potentiometers 73, 75~ 77 serve for ad~usting the sensitivity of the trigger circuitsO

~he outputs of the two level detector circuits 74 and 76 are connected to respective inputs o~ a ~AND gate 78 . whose output is connected to one input of a further gate 80.
The circuit 72 is connected to a second input of ~A~D
gate 80 via an inver-ting amplifier 820 Amplifiers 68 and 70 for sidebands fa and f~ are also con~ected to respective level detector ~lrcuits 84 and 86 designed to respond to relativel~ high le~el input signals to switch their outputs from logic 1 to logic 0 . signals. Potentiometers 85 and ~7 also ser~e for , _ 19_ ... --_ . . .. .. . . i, .. . .

., ; . .. .
.~ : . - : :. ~

' ': ~ "; :'.' ' ,.,",, ', '726 ad~usting the sensitivity Or the level detec tor circuits 84 and 86. ~he outputs o~ the circuits 84~ 86 ars connected to respective inputs of a ~ND gate 8B whose output is connected via an inverting amplifier 89 to one input of a NAND gate 90~ The other input o~ NEND gate 90 is connected to the output of ~AN]D gate 80 and it~
output is connected to war~ing device 92.

Assuming the marker tag 40 passes close to one o~ the induction band transmitter aerials, for example aerial 36, the field stre~gth Or signal fa at the tag 40 will be large thus producing a high depth o~ modulation of the carrier fc by fa The level of ~ig~al fa thus detected by the receiver a~d applied to the trigger circuits 74 and 84 would be high and exceed both the low and high level switching thresholds of the trigger circuits 74 and 84. The.output of the latter would thus be at logic 0. q`he logic 0 output of the trigger circuit 84 would result in a logic 0 signal applied to one input of NAND gate 90 via NA~D gate 88 and inverter 89. This would generate a logic 1 signal at the output of NAND
gate 90 to activate the warning device 92~ ~bis xesult would not be afrected by the state of the outputs of the .
trigger circuits for signals fb and (fa ~ fb~' . .
If the tag 40 passes close to aerial ~8 the logic circuit would operate in a similar manner, the warning device 92 being activated via ~A~Dgates 88~ 90 and in~erter ~9 as a result of the i~ten~ity of the received ~b signals.
. - .
However, if the tag 40 i8 i~troduced ints the zoDe 34 _ 20_ ", . . . ..
,, , 38 the various ~ideband ~ignal~ would be closer iB
a~plitude and OI lower ~nten~ityO The trigger circuit~
84 and 86 wo~ld then o~ course rema:in unswitc~ed ~ generatirlE;
logic 1 output~ and ~ loE~;ic 1 ~igna:l at one input o~
t~e ~A~D .gate 90. ~herefore ~or the latter to activate the warning de~ice 9 the low level ltrigger circuits 72, 74 and 76 must be switched i~ the combi~atio~ or - com~inations to produce a logic C) ~ignal at the other i~put of ~A~D gate 90. In the illustrated circuit thi~ . . .
requires a combination OI low level signals fa or fb ith Cfa ~ f~ i~Qal fa alone~ fb alone or (f ~ ~b) ~, alone i8 insufficient to activat~ the warning devic2.
he logic circuit ma;sr be e~panded and modified 1;o make use of further inter modulatio~ product~ and further reduoe the ~ensitivit~ of the ~ystem to false alarm~;

A logic table for the logic cirouit o~ ~lgure 3 is ~, giv~n below:

:

.1 liO';26 .
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o ~ o ~

$ ~ . . .
o o ~ ~ ~ ~
.
,. CO ' CO ~ ~ o . ~ o C~ .

.
o o~ ~ o ~ o .
~ , . .
a:) o ~ o : ~ ~ o ~ o ~ C o .~, , ~.

. ~ooooo ,; , :
~ ~ ~ ~ o o o . , ~ 0 .~ ~ '~ ~ ~ ~ C~ o .~ :

.
.. ,_. ~

~ +

~ .
o ~ ~ ~2 ,, . . "... .. . ..
.: . ~ . . . . .
, .. , :, ,.
~. . . ..... . .
; : , , . ;. ' :, : :
, ~ ;........ . .. :, .

1~ f~

~he trigger st~ges 72, 74/ 76~ 84 and 88 ma~ includ~
detection and smoothing circuits to provide d.c voltage~
proportional to the amplitude of the input ~ignals.

In order to obtain an i~dication of the relati~e location of the tag 40 withi~ the colume 34 the amplitudes of signals fa and f~ are compared in a differential a~plifier 100 and the resulting comparison sig~al utilised to energi~e ~isual indicators such a~ lamps 102 to ~10 ~epresenting intervals of distance between the aerials 36.and 38. The output of the amplifier 1Q0 may for exampl0 be in the form of a varying doc signal which is used to trigger various switchi~g circuits 112 to 120 having progressively increasing switching thresholds~ Although only five lamps are illustrated the positional indication can be made as cor~rse or as ~ine as desired by varying ~he number of lamps and switching circuitsO The visual indicators may be replaced by a~ ~uaible indicator, the dif~erent possible position~ of the tag bei~g represented by different audibls frequencies, either discrete or ~o continuously ~ariable.
r As an alternative to the use of a differential amplifier 100 or as an i~itial, coarse positional indicator the signals fa and fb could be utilised to activate respective visu~l or audible indicators whenever a certain signal ~; 25 threshold were exceeded. This would cater for the ends of the volume 34 while the signal (fa ~ fb) could be used to indicat~ a more central position where a strong - composite signal (f~ ~ fb) would be expected.

- 2~ -2~6i Intermediate position~ may be identified by combination~
of the three ~ignal strength~ monitored ~y a suitable logic circuit which controlfi appropriate visual and/or audibls indicatorsO The system oî Figure ~ could readil~ be ad~usted for this purpose by connectinæ lamps to trigger circuits 84 and 86 ~nd NhND gate 82, as indicated by arrows, the first two serving respectively to indicate extremes of the zone 34 and the third, the central region of zo~e 34. ~ .
' ' , , .

One:advanta~e of the present s~stem when the latter is used to monitor a vertical area much as a doorway is described below. As a tag iB brought toward~ the area, initiall~ the difference in the distances of the tag from the two transmitter aeri.als is small compared to the actual distances and the difference in field strengths of the two signals fa and fb at the tag is ne~ligible. ~he receiver thus indicates a central disposition of the tag~ However, as the tag i5 brought closer, for example to pass close to aerial 36, the difference in field strengths of the two signals increase~ in significance to am~ximum at the tag's shortest distance from the transmitters~ A~ this difference in field strengths increases, and then decreases again once the tag has passed through the ; 25 doorway, the receiver indicates a change in tag position from a central position to an extreme position and then back to a central position. It is therefore possible ~o determine 7 with accuracy not onl~ the ; ' . ' ` , '~
' :' , ';~ ; ' ~;
.
~, , ` ' . , ,:

~ , ..

position of the tag in the doorwa~ but the exact moment the tag is in the doorwayO

~he system of Figure ~ may be further improved as show~
i~ chain lines by amplitude modulating the tran~mitted frequencie~ fa~ fb with a to~e frequenc~ ~ preferably in the range 10Hz to 10 ~ 9 by means of a modulator 122.
~his tone ~m can then be recovered from the signa'Læ
f~ fb and (fa ~ fb) b~ suitable filters 124, 126, ~
128 in the lo~ic circuit. This facilitates di~orimination o~ weak signals from tags at considerable range.f:rom background noise~ A number of different zones 34 may be controlled from the same three remote transmitters 30, 32 and 42 without interf~rence proving a problem if a different modulation tone is used in each case~

~urther improvement in the systems abilit~ to disti~guish ge~uine signals from noise may be obtained b~ compari~g both phase and frequen~y of the transmitted signals fa~
fb~ ~f ~ fb) with the received si gal~, or of the modulation tone filtered through filters 124 and 128 with the original modulating tone~ A modification of Figure 3 is shown in dotted lines where respective gating circuit~ 130, 132 and 134 are connected to the outputs of filters 124, 126 a~d 128, one input of each cirGuit 130q 132, 1~4 being connected to the modulator 122 such that signals from the filter~ 124 to 126 are only passed to the trigger circuits 72 to 76 when both phase and frequenc~ coincide ~ith the modulation si~n~ls ~` from the modulator 122.

7~
A further modification Or the sy~tem of ~igure ~ i8 . shown i~ ~i~ur~ 5. ~his modification allow~ triggering ol the warning device 92 only after a ta~ is prese~t in the zone ~4 for apreaelected time~ The output~ o~ the . 5 modulator 122 and the filters 124~ 126 and ~28 are e~ch co~nected to a fir~t input o~ a respective comparato~
140~ 142, 144, 146 a reference volt~ge 80urc~ being connected to the second i~put thereofa ~ach comparator . i~ coD~ected by w~y o~ a re~pective divider circuit ~ ^
148 to 154 for example a divide-by-~en circuit? to a ~CD decoder 156 to 162. ~he output of decoder 156 iæ
. connected ~ia a negating circuit 164 to reset inp!lt~
of the divider circuits 150 to 54O ~he decoders 158 to 162 are set to pro ~ e an output signal at the eighth input pulse to the divider circuit~ 150 to 154 while decoder 156 is set to provide an output signal at the ninth input pulse to divider 148. (~hese counts may be varied a~ desired provided the count of decoder 156 i8 greater ~- than those o~ decoder~ 158, 160 and 162.) ,, .
~ach cycle of the modulating frequency fm generat~s a pulse at the output of comparator 140 which is applied .to divider circuit 148. ~he decoder 156, at the ni~th such successive pulse, resets the di~iders 158 to 162D
W~ere the input sig~als to comparators 142, 144 a~d 146 are random noise signals or wea~ intermittent modulation : tone pulses the dividers 158 to 162 will be ~upplying an output pulse at the eighth input pulse to divider~
150 a~d 154D However, where the input sig~al to on~
.~ .

_ 26 -, , . .: ' . ':

3'7~:6 or more of the comparators 142~ to 146 is a conti~uous modulation tone (indicRking ~he presence of a ta~ 40 in the volume 34) then the associ~ted decode~ 'l58, 160, 162 generates an output pu~se before it can be reset b~ the decoder 156~ ~he outputs of the decoders 158 to 162 are connected to the warning device 92 by w~y of a logic circuit such as that shown in ~igure 3 which activates the alarm for one or more desired combinations o~ outp~t signals from counters 158, 1~0 a~d 162.

~inally, although the system described with re~erence to Figure 3 uses the induction band frequencie~, frequencies in the MegaHert~ range, e.g~ 13.5 ~ may be used.

An automatic check for the system of the present invention may be provided by permanently locating in the zone a tag whose non-linear eleme~t is for example a diode which is inactive until stimulated by suitable means~ A light responsive diode coupled via a fibre optic system to a light source which is periodically energised by the system for a short time, for example one second each ten minutes.
At the same time the diocle is activated the system can also activate a suitable indicator to show that the system is on test.
.. . .

. .

~^ 27 :: :,,, :
,~ :
. . .
. : . . :,:

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVELEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of monitoring the position in a surveillance zone of an electromagnetic wave receptor reradiator with signal mixing capability, comprising the steps of radiating simultane ously first and second energy fields through said zone for causing said receptor reradiator to radiate at least one reply signal which is a function of said energy fields and of the position of the receptor reradiator in the zone and wherein said energy fields are established respectively from spaced apart locations adjacent the edges of said zone, said first energy field is produced by a first high frequency signal and said second energy field is produced by a second high frequency signal of a different frequency to said first high frequency signal;
detecting in said zone the presence of said reply signal;
and indicating the position of the receptor reradiator in the zone and triggering an alarm in response to the detection of said reply signal.

2. A method as claimed in claim 1 further comprising the steps of modulating at least one of said first and second signals with a modulation signal to modulate the corresponding energy field whereby said receptor reradiator radiates a reply signal which has a wave envelope determined by said modulation; and comparing the frequency and phase of the wave envelope of the reply signal with the modulation signal and triggering the alarm dependence on the comparison.

3. A surveillance system for monitoring the position in a surveillance zone of an electromagnetic wave receptor reradiator with signal mixing capability, comprising in combination: a source of a first high frequency signal;
a first means coupled to said source for radiating through said zone a first energy field corresponding to said first high frequency signal; a source of a second high frequency signal of a different frequency to said first high frequency signal; second means coupled to said source for radiating through said zone a second energy field corres-ponding to said second high frequency signal, said first and second means establishing said energy fields respectively from spaced apart locations adjacent the edges of said zone;
a receptor reradiator operable to detect said energy fields and to radiate at least one reply signal which is a function of said signals and of the position of said receptor reradiator in the zone; a receiver for detecting said reply signal, means controlled by the receiver in dependence upon detection of said reply signal to indicate the position of the receptor reradiator in the zone; and an alarm coupled to the receiver for providing an alarm signal responsively to the receiver detecting the reply signal.

4. A system as claimed in claim 3 wherein the receiver includes means for comparing said reply signal with a reference signal and controlling the position indicating means in dependence upon the comparison.

5. A system as claimed in claim 3 further comprising modulator means for modulating at least one of said first and second signals with a modulation signal whereby to cause said receptor reradiator to radiate a reply signal which has a wave envelope determined by said modulation; and said receiver includes a logic circuit operable to compare the frequency and phase of the wave envelope of said reply signal with the modulation signal and to trigger the alarm in dependence on the comparison.

6. A system as claimed in claim 3 wherein the reply signal comprises a sum of the frequencies of said first and second signals.

7. A system as claimed in claim 3 wherein the reply signal comprises the difference between the frequencies of said first and second signals.

8. A system as claimed in claim 6 wherein said first and second signals are respectively 27 MHz and 27.2 MHZ.

9. A system as claimed in claim 3 wherein the frequencies of said first and second signals are spaced apart such that the reply signal is a function of the higher frequency signal of said first and second signals modulated by the lower frequency signal thereof.

10. A system as claimed in claim 9 wherein the receiver includes means to compare the depth of modulation with a reference signal and control the position indicating means in dependence upon the comparison.

11. A system as claimed in claim 10 wherein said reference signal is the lower frequency signal of the first and second signals, which lower frequency signal is the modulating signal.

12, A method of detecting the presence in a survillance zone of an electromagnetic wave receptor reradiator with signal mixing capability, comprising the steps of simultan-eously radiating first, second and third energy fields through said zone for causing said receptor reradiation to radiate at least one reply signal which is a function of said energy fields,wherein said first energy field is produced by a microwave signal and said second and third energy fields are produced by low frequency signals relative to said microwave signal;detecting in said zone the presence of said reply signal; and triggering an alarm in response to detection of said reply signal.

13. A method as claimed in claim 12 wherein said second and third energy fields are radiated into said zone at spaced apart locations adjacent the edges of said zone, and said reply signal is a function of the position of said receptor reradiator and the method further comprises the step of indicating the position of the receptor reradiator in the zone.

14. A surveillance system for detecting the presence in a surveillance zone of an electromagnetic wave receptor reradiator with signal mixing capability comprising in combination: a source of a first, microwave signal;
means coupled to said source for radiating through said zone a first energy field corresponding to said microwave signal; a source of a second signal; means coupled to said source for radiating through said zone a second energy field corresponding to said second signal; a source of a third signal;means coupled to said source for radiating through said zone a third energy field corresponding to said third signal, wherein said second and third signals are at different,low frequencies relative to the microwave signal; a receptor reradiator operable to detect said energy fields and to radiate at least one reply signal which is a function of said signals; a receiver for detecting said reply signal; and an alarm coupled to the receiver for providing an alarm signal responsively to the receiver detecting the reply signal.

15. A system as claimed in claim 14 wherein said means radiating said second and third energy fields are positioned respectively at spaced apart locations adjacent the edges of said zone and said reply signal is a function of the position of said receptor; and the said system further comprises means controlled by the receiver in dependence upon detection of said reply signal to indicate the position of the receptor reradiator in said zone.

16. A system as claimed in claim 15 wherein two reply signals serve for indicating the position of the receptor reradiator in said zone, one of which said reply signals comprises said first signal modulated by said second signal and the other of which comprises said first signal modulated by said third signal.

17. A system as claimed in claim 16 wherein the receiver includes means coupled to said source of microwave signal for separating said second and third signals from said first signal.

18. A system as claimed in claim 17 wherein the receiver includes means to compare the amplitude of said second and third signals one with the other and control said position indicating means in dependence on the comparison.

19. A system as claimed in claim 15 wherein the alarm means is operable to provide said alarm signal in response to the receiver detecting at least one of said second and third signals as modulation on said microwave signal.

20. A system as claimed in claim 19 wherein the receiver comprises a logic circuit operable to trigger said alarm responsively to at least one of the second and third signals detected by the receiver exceeding a first preselected threshold.

21. A system as claimed in claim 20 wherein the receiver is operable to detect said second and third signals and an inintermodulation product of said second and third signals when received as modulation on said microwave signal and comprises a logic circuit operable to trigger the alarm responsively to said intermodulation product and one of said second and third signals exceeding a second preselected threshold less than said first preselected threshold.

22. A system as claimed in claim 15 wherein said second and third signals lie in the range 16KHz to 150 KHz.

23. A system as claimed in claim 15 further comprising means for modulating said second and third signals with a tone signal whereby to cause the receptor reradiator to radiate a reply signal which has a wave envelope determined by said tone signal, and the receiver includes means to compare the wave envelope of the reply signal with said tone signal and trigger said alarm only when said envelope and said output are matched.